This invention relates to a paper bag for in particular fine-grained bulk material such as cement, gypsum, granular material, animal feed or the like, with a bottom and with an upper part opposite the bottom, wherein bottom and/or upper part are folded and glued by closing up the side parts around side fold lines, in particular in the form of a cross or block bottom. Another aspect of the invention relates to a special manufacturing method for such paper bag.
Generic paper bags are known, for example from EP 1 858 769 B1. Widespread in the trade in particular are the usual sizes of 5 kg, 10 kg or 25 kg. They have one or more paper layers which are formed of paper or a paper compound and/or coated paper. Layers of other materials also can be present, for example a plastic film like in the case of EP 1 858 769 B1.
Such paper bags generally are manufactured from endless paper webs whose longitudinal edges are glued together to form a tube. To form the bags, the tube is cut into portions of suitable length and at the open ends of the tube portions bottom and upper part are folded and glued, in order to close the paper bag at the bottom and/or at the top. Usually, there is chosen a cross or block bottom folding, wherein in general a creasing or the like is made to facilitate folding in the region of the fold lines. The folded parts are glued together by means of a glue, usually a starch-water mixture. For manufacturing reasons, however, the used glue must be left out in the region of the fold lines. This restriction is due to the fact that in the region of the fold lines and in the edge region the glue would lead to soiling of the production machine for the manufacture of paper bags and a continuous production of the paper bags would thus not be possible. In particular, when applying a creasing in the region of the fold lines, a glue application present in this region would lead to strong soiling of the tools used in the machine for creasing. In addition, when applying the starch-glue pattern a certain minimum distance from the edges or lateral edges of the bottom/upper part also must be maintained, as otherwise soiling and gluing together of certain machine parts likewise is likely to occur.
Due to the omission of glue, however, a non-glued region remains along the fold lines and edges/lateral edges in the bottom and/or upper part of the paper bag. This may produce channels which provide for the exit of the dust-like filling material also after closing of the bag.
Furthermore, closing up the bottom/upper part effects a certain compression of the paper, whereby undulations of the paper as well as folds and displacements are obtained in the region of the abutting edges of the center of the bottom, also referred to as bottom center line. These undulations, folds and edges also cannot be glued completely with the starch glue, as the same does not enter completely and does not reach up to the folded edge. This leads to an additional channeling beside the channels of the fold lines, which increase the exit quantity of the filling material.